A bifocal contact lens having a circular near-vision zone formed in its central region coaxially with its optical axis, and an annular distance-vision zone formed around and concentric with the near-vision zone is proposed in, for example, JP-A No. 60-91327.
Referring to FIG. 12, a contact lens 1 has a front surface consisting of a near-vision zone N for near vision and a distance-vision zone F for distance vision, and a base surface 3 corresponding to the curved surface of the wearer's cornea. When the lens is fitted over a central part of the cornea of the eye, the near-vision zone covers substantially half the pupil of the eye under an average reading light condition about 80 ft (about 24.4 m) candle (80 ft-candle) by definition.
A person wearing a contact lens meeting such a condition is able to use a distance-vision zone and a near-vision zone appropriately and smoothly by using the distance-vision zone and the near-vision zone intentionally selectively. However, the optical center of the contact lens specified for the person often does not- coincide with the center of the person's pupil, which makes the appropriate selective use of the distance-vision zone and the near-vision zone difficult. A multifocal contact lens has been proposed, for example, in JP-A No. 7-239459 to solve such a problem.
FIG. 13 shows a contact lens 1 having a front surface 2 consisting of a near-vision zone N and a distance-vision zone F, and a base surface 3 corresponding to the curved surface of the wearer's cornea. This contact lens is a multifocal lens having a near-vision zone (of a diameter in the range of 0.8 to 3.5 mm) including an optical axis and decentered toward the nose by a distance in the range of 0.2 to 2.4 mm from a vertical longitude passing the geometrical center of the contact lens. This contact lens 1 is provided with a prism ballast and a peripheral part thereof is slabbed off to prevent the contact lens 1 from turning and to position the same correctly on the eye. When the contact lens 1 shown in FIG. 13 is fitted over the eye, the contact lens 1 can be positioned with the optical axis passing the center of the near-vision zone substantially coincided with the center of the pupil of the eye.
Multifocal contact lenses capable of preventing spherical aberration are disclosed in, for example, JPA- No. 5-508019 and its U.S. counterpart 5,541,578.
These prior art multifocal contact lenses have an alternate concentric arrangement of distance-vision zones and near-vision zones.
A contact lens 1 shown in FIG. 14, one of those lenses, has a front surface having an alternate, concentric arrangement of distance-vision zones F1. F2 . . . for distance vision, and near-vision zones N1, N2, . . . for near vision. In this lens, the distance-vision zones F1, F2, . . . are arranged so that light rays parallel to the optical axis of the lens 1 and falling on the distance-vision zones F1, F2, . . . may be focused substantially on the same point on the optical axis.
A concentric progressively variable power multifocal contact lens is proposed in, for example, Hisao Magariya, "Roshi-yo Kontakuto Renzu (Contact Lens for Presbyopia)", Atarashii Ganka 10, pp. 1543-1544 (1995).
A contact lens 1 shown in FIG. 15 has a front surface 2 having a single spherical surface, and a base surface 3 having the shape of an inside hyperbolic aspherical surface. A central region of the base surface 3 has a shape for distance vision, and power for near vision increases progressively from a middle region toward a peripheral region. Since the base surface 3 flattens sharply from the apex toward the edge thereof, the curvature of a region around the apex of the base surface 3 is greater than that corresponding to the radius of curvature of the cornea.
There are various contact lenses having both near-vision zones and distance-vision zones as those mentioned above.
Practical contact lenses must meet the following practical requirements. However, the prior art contact lenses do not necessarily meet those requirements satisfactorily.
A contact lens fitted over the cornea repeats horizontal and vertical motions (hereinafter referred to as "decentering motions") stopping at a stabilizing position, and turning on the cornea (hereinafter referred to as "turning motions") every time the wearer blinks.
These motions including the decentering motions of the contact lens on the eye contribute to discharging body wastes collected between the contact lens and the cornea outside together with tears and supplying oxygen to the cells of the cornea together with tears. Thus, the motions of the contact lens on the eye provides important physiological functions.
The decentering motion dislocates the center of the contact lens from a position corresponding to the center of the pupil. The decentering motion is not any significant problem for a single-focus contact lens. Since the multifocal contact lens has an alternate arrangement of annular near-vision zones and annular distance-vision zones, decentering motions are impediments to providing a clear vision.
However, there has been no direct mention on how the positional relation between the distance-vision zones and the near-vision zones of a multifocal contact lens must be to give high vision regardless of decentering motions.
The size, i.e., the diameter, of the pupil of the eye of a man wearing a contact lens varies autonomically according to the brightness of the ambiance for important physiological functions.
Since a multifocal contact lens has an alternate arrangement of annular near-vision zones and annular distance-vision zones, the distribution ratio between the near-vision zones and the distance-vision zones in the pupil varies with the variation of the diameter of the pupil and, consequently, the distribution ratio between distance-vision performance and near-vision performance of the contact lens varies according to the brightness of the ambiance.
The variation of the diameter of the pupil, together with decentering motions, causes the more complex variation of the distribution ratio between the near-vision zones and the distance-vision zones in the pupil.
However, there has been no direct mention on how the positional relation between the distance-vision zones and the near-vision zones of a multifocal contact lens must be to give high vision regardless of the variation of the diameter of the pupil.